The objective of this project is to use mechanism based screens to identify novel estrogen receptor (ER) modulators which can be used as breast cancer chemo-preventatives and chemotherapeutics. During the past funding funding period we developed and applied a series of in vitro models which permitted the identification of compounds which functioned as anti-estrogens in the breast but which did not exhibit uterotrophic activity. This led to the identification of GW5638, a drug which functions as a pure anti-estrogen in rat models which are predictive of estrogenic activity in the human endometrium. Interestingly, GW5638 functions as a pure anti-estrogen in estrogen dependent breast tumors in a-thymic nude mice and, like tamoxifen, functions as an agonist in bone and the cardiovascular system. Because of these unique properties and it superior pharmaceutical properties, we intend to introduce GW5638 into the clinic. Consequently, we propose in this grant a series of experiments aimed at defining the clinical utility of the compound. Specifically, we will examine the in vivo pharmacological activity of the novel anti-estrogen GW5638 in each of three ways: (A) Analysis of the activity of GW5638 in MCF-7 cell tumor xenografts which are resistant to tamoxifen. (B) Comparison of the time to resistance/failure of MCF-7 cell tumor xenografts when treated with GW5638 or tamoxifen. (C) In tumors which are resistant to GW5638 we intend to determine if they are cross-resistant to tamoxifen and the pure anti-estrogen ICI182,780 The surprising finding that all the anti-estrogens tested thus far, with the exceptions of ICI182,780, are capable of functioning as ER agonists in bone encouraged us to seek alternative pathways by which these compound compounds, through ER, could manifest their unique biology. In this light we, and other, have shown that estradiol in an ER-dependent manner can activate mitogen activated protein kinase (MAPK). This original observation has been extended to show that all ER ligands, except the pure anti-estrogen ICI182,780, behave similarly. Thus we hypothesize that this pathway is an important component of estrogen signaling and that it proves a potential mechanism by which cells can become resistant to the actions of anti-estrogens. We intend to probe this hypothesis be: (A) Evaluating the potential synergy of anti-estrogens and farnesyl transferase inhibitors in tamoxifen responsive breast cancers. (B) Evaluating the activity of farnesly transferase inhibitors in tumors which have become unresponsive to tamoxifen (and/or GW5638) and (C) By applying genetic and pharmacological methodologies we hope to define the role of ER in MAPK activation in breast cancer cells. It is anticipated that this project, as outlined, will facilitate a smooth passage of GW5638 to the clinical and permit it to be tested in as broad a clinical setting as possible.